Principles of So3ware Construc9on: Objects, Design, and Concurrency. Introduc9on to Java. Josh Bloch Charlie Garrod Darya Melicher

Transcription

1 Principles of So3ware Construc9on: Objects, Design, and Concurrency Introduc9on to Java Josh Bloch Charlie Garrod Darya Melicher 1

2 Administrivia Homework 1 due next Thursday 11:59 p.m. Everyone must read and sign our collabora9on policy First reading assignment due Tuesday Effec9ve Java Items 15 and 16 2

3 Outline I. Hello World! explained II. The type system III. Quick n dirty I/O IV. Collec9ons V. Methods common to all Objects 3

4 The simplest Java Program class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 4

5 Complica9on 1: you must use a class even if you aren t doing OO programming class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 5

6 Complica9on 2: main must be public class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 6

7 Complica9on 3: main must be static class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 7

8 Complica9on 4: main must return void class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 8

9 Complica9on 5: main must declare command line arguments even if unused class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 9

10 Complica9on 6: standard I/O requires use of static field of System class HelloWorld { public static void main(string[] args) { System.out.println("Hello world!"); 10

11 Execu9on is a bit complicated First you compile the source file javac HelloWorld.java Produces class file HelloWorld.class Then you launch the program java HelloWorld Java Virtual Machine (JVM) executes main method 11

12 On the bright side Has many good points to balance shortcomings Some verbosity is not a bad thing Can reduce errors and increase readability Modern IDEs eliminate much of the pain Type psvm instead of public static void main Managed run9me has many advantages Safe, flexible, enables garbage collec9on It may not be best language for Hello World But Java is very good for large-scale programming! 12

13 Outline I. Hello World! explained II. The type system III. Quick n dirty I/O IV. Collec9ons V. Methods common to all Objects 13

14 Java type system has two parts Primi/ves int, long, byte, short, char, float, double, boolean No iden9ty except their value Immutable On stack, exist only when in use Object Reference Types Classes, interfaces, arrays, enums, annota9ons Have iden9ty dis9nct from value Some mutable, some not On heap, garbage collected Can t achieve unity of expression Unity of expression with generics Dirt cheap More costly 14

15 Programming with primi9ves A lot like C! public class TrailingZeros { public static void main(string[] args) { int i = Integer.parseInt(args[0]); System.out.println(trailingZerosInFactorial(i)); static int trailingzerosinfactorial(int i) { int result = 0; // Conventional name for return value while (i >= 5) { i /= 5; result += i; return result; // Same as i = i / 5; Remainder discarded 15

16 Primi9ve type summary int 32-bit signed integer long 64-bit signed integer byte 8-bit signed integer short 16-bit signed integer char 16-bit unsigned integer/character float 32-bit IEEE 754 floa9ng point number double 64-bit IEEE 754 floa9ng point number boolean Boolean value: true or false 16

17 Deficient primi9ve types byte, short use int instead! byte is broken should have been unsigned float use double instead! Provides too lijle precision Only compelling use case is large arrays, especially in resource-constrained environments 17

18 The class hierarchy The root is Object (all non-primi9ves are objects) All classes except Object have one parent class Specified with an extends clause class Guitar extends Instrument {... If extends clause omijed, defaults to Object A class is an instance of all its superclasses Object Instrument Toy Guitar Yoyo 18

19 Implementa9on inheritance A class: Inherits visible fields and methods from its superclasses Can override methods to change their behavior Overriding method implementa9on must obey contract(s) of its superclass(es) Ensures subclass can be used anywhere superclass can Liskov Subs9tu9on Principle (LSP) 19

20 Interface types Defines a type without an implementa9on Much more flexible than class types An interface can extend one or more others A class can implement mul9ple interfaces interface Comparable<T> { /** * Returns a negative number, 0, or a positive number as this * object is less than, equal to, or greater than other. */ int compareto(t other); 20

21 Enum types Java has object-oriented enums In simple form, they look just like C enums: enum Planet { MERCURY, VENUS, EARTH, MARS, JUPITER, SATURN, URANUS, NEPTUNE But they have many advantages! Compile-9me type safety Mul9ple enum types can share value names Can add or reorder without breaking exis9ng uses High-quality Object methods are provided Screaming fast collec9ons (EnumSet, EnumMap) Can iterate over all constants of an enum 21

22 Boxed primi9ves Immutable containers for primi9ve types Boolean, Integer, Short, Long, Character, Float, Double Lets you use primi9ves in contexts requiring objects Canonical use case is collec/ons Don t use boxed primi/ves unless you have to! Language does autoboxing and auto-unboxing Blurs but does not eliminate dis9nc9on There be dragons! 22

23 Pop Quiz! 23

24 What does this fragment print? int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ; int i; int sum1 = 0; for (i = 0; i < a.length; i++) { sum1 += a[i]; int j; int sum2 = 0; for (j = 0; i < a.length; j++) { sum2 += a[j]; System.out.println(sum1 - sum2); 24

25 Maybe not what you expect! int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ; int i; int sum1 = 0; for (i = 0; i < a.length; i++) { sum1 += a[i]; int j; int sum2 = 0; for (j = 0; i < a.length; j++) { // Copy/paste error! sum2 += a[j]; System.out.println(sum1 - sum2); You might expect it to print 0, but it prints 55 25

26 You could fix it like this int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ; int i; int sum1 = 0; for (i = 0; i < a.length; i++) { sum1 += a[i]; int j; int sum2 = 0; for (j = 0; j < a.length; j++) { sum2 += a[j]; System.out.println(sum1 - sum2); // Now prints 0, as expected 26

27 But this fix is far bejer int sum1 = 0; for (int i = 0; i < a.length; i++) { sum1 += a[i]; int sum2 = 0; for (int i = 0; i < a.length; i++) { sum2 += a[i]; System.out.println(sum1 - sum2); // Prints 0 Reduces scope of index variable to loop Shorter and less error prone 27

28 This fix is bejer s9ll! int sum1 = 0; for (int x : a) { sum1 += x; int sum2 = 0; for (int x : a) { sum2 += x; System.out.println(sum1 - sum2); // Prints 0 Eliminates scope of index variable en/rely! Even shorter and less error prone 28

29 Lessons from the quiz Minimize scope of local variables [EJ Item 57] Declare variables at point of use Ini9alize variables in declara9on Use common idioms Watch out for bad smells in code Such as index variable declared outside loop 29

30 Outline I. Hello World! explained II. The type system III. Quick n dirty I/O IV. Collec9ons V. Methods common to all Objects 30

31 Output Unformajed System.out.println("Hello World"); System.out.println("Radius: " + r); System.out.println(r * Math.cos(theta)); System.out.println(); System.out.print("*"); Formajed System.out.printf("%d * %d = %d%n", a, b, a * b); // Varargs 31

32 Command line input example Echos all command line arguments class Echo { public static void main(string[] args) { for (String arg : args) { System.out.print(arg + " "); $ java Echo Woke up this morning, had them weary blues Woke up this morning, had them weary blues 32

33 Command line input with parsing Prints GCD of two command line arguments class Gcd { public static void main(string[] args) { int i = Integer.parseInt(args[0]); int j = Integer.parseInt(args[1]); System.out.println(gcd(i, j)); static int gcd(int i, int j) { return i == 0? j : gcd(j % i, i); $ java Gcd

34 Scanner input Counts the words on standard input class Wc { public static void main(string[] args) { Scanner sc = new Scanner(System.in); long result = 0; while (sc.hasnext()) { sc.next(); // Swallow token result++; System.out.println(result); $ java Wc < Wc.java 32 34

35 Outline I. Hello World! explained II. The type system III. Quick n dirty I/O IV. Collec9ons V. Methods common to all Objects 35

36 Primary collec9on interfaces Collection Map Set List Queue Deque 36

37 Primary collec9on implementa9ons Interface Set List Queue Deque [stack] Map Implementa/on HashSet ArrayList ArrayDeque ArrayDeque ArrayDeque HashMap 37

38 Other noteworthy collec9on impls Interface Set Queue Map Implementa/on(s) LinkedHashSet TreeSet EnumSet PriorityQueue LinkedHashMap TreeMap EnumMap 38

39 Collec9ons usage example 1 Squeeze duplicate words out of command line public class Squeeze { public static void main(string[] args) { Set<String> s = new LinkedHashSet<>(); for (String word : args) s.add(word); System.out.println(s); $ java Squeeze I came I saw I conquered [I, came, saw, conquered] 39

40 Collec9ons usage example 2 Print unique words in lexicographic order public class Lexicon { public static void main(string[] args) { Set<String> s = new TreeSet<>(); for (String word : args) s.add(word); System.out.println(s); $ java Lexicon I came I saw I conquered [I, came, conquered, saw] 40

41 Collec9ons usage example 3 Print index of first occurrence of each word class Index { public static void main(string[] args) { Map<String, Integer> index = new TreeMap<>(); // Iterate backwards so first occurrence wins for (int i = args.length - 1; i >= 0; i--) { index.put(args[i], i); System.out.println(index); $ java java Index if it is to be it is up to me to do it {be=4, do=11, if=0, is=2, it=1, me=9, to=3, up=7 41

42 More informa9on on collec9ons For much more informa9on on collec9ons, see the annotated outline: hjps://docs.oracle.com/javase/8/docs/technotes /guides/collec9ons/reference.html For more info on any library class, see javadoc Search web for <fully qualified class name> 8 e.g., java.util.scanner 8 42

43 What about arrays? Arrays aren t really a part of the collec9ons framework But there is an adapter: Arrays.asList Arrays and collec9ons don t mix Arrays are covariant and reified Generics are nonvariant and erased If you try to mix them and get compiler warnings, take them seriously Generally speaking, prefer collec9ons to arrays See Effec;ve Java Item 28 for details 43

44 Outline I. Hello World! explained II. The type system III. Quick n dirty I/O IV. Collec9ons V. Methods common to all Objects 44

45 Methods common to all objects How do collec9ons know how to test objects for equality? How do they know how to hash and print them? The relevant methods are all present on Object equals - returns true if the two objects are equal hashcode - returns an int that must be equal for equal objects, and is likely to differ on unequal objects tostring - returns a printable string representa9on 45

46 Object implementa9ons Provide iden;ty seman;cs equals(object o) returns true if o refers to this object hashcode() returns a near-random int that never changes over the object life9me tostring() returns a nasty looking string consis9ng of the type and hash code For example: java.lang.object@659e0bfd 46

47 Overriding Object implementa9ons No need to override equals and hashcode if you want iden/ty seman/cs When in doubt, don t override them It s easy to get it wrong Nearly always override tostring println invokes it automa9cally Why sejle for ugly? 47

48 Overriding tostring Overriding tostring is easy and beneficial final class PhoneNumber { private final short areacode; private final short prefix; private final short public String tostring() { return String.format("%03d-%03d-%04d", areacode, prefix, linenumber); Number jenny =...; System.out.println(jenny); Prints:

49 Overriding equals Overriding equals is tricky here s the contract The equals method implements an equivalence rela/on. It is: Reflexive: For any non-null reference value x, x.equals(x) must return true. Symmetric: For any non-null reference values x and y, x.equals(y) must return true if and only if y.equals(x) returns true. Transi/ve: For any non-null reference values x, y, z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) must return true. Consistent: For any non-null reference values x and y, mul9ple invoca9ons of x.equals(y) consistently return true or consistently return false, provided no informa9on used in equals comparisons on the objects is modified. For any non-null reference value x, x.equals(null) must return false. 49

50 Overriding hashcode Overriding hashcode also tricky here s contract Whenever it is invoked on the same object more than once during an execu9on of an applica9on, the hashcode method must consistently return the same integer, provided no informa9on used in equals comparisons on the object is modified. This integer need not remain consistent from one execu9on of an applica9on to another execu9on of the same applica9on. If two objects are equal according to the equals(object) method, then calling the hashcode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(object) method, then calling the hashcode method on each of the two objects must produce dis9nct integer results. However, the programmer should be aware that producing dis9nct integer results for unequal objects may improve the performance of hash tables. 50

51 Why the contracts majer No class is an island If you put an object with a broken equals or hashcode into a collec9on, the collec9on breaks! Arbitrary behavior may result! System may generate incorrect results or crash To build a new value type, you must override equals and hashcode Next lecture we ll show you how 51

52 Summary Java is well suited to large programs; small ones may seem a bit verbose Bipar9te type system primi9ves & object refs Single implementa9on inheritance Mul9ple interface inheritance A few simple I/O techniques will get you started Collec9ons framework is powerful & easy to use 52